I downloaded Eric's gcc-propeller project. Now how do I build it on my Linux machine? It's been years since I built PropGCC, and it was under Cygwin. I recall having to run some type of config tool first, and then running make, but I can't figure out which directory to do this from.

Try cloning my propeller-gcc project. It has a README.md file that says how to make it. It will build gcc, binutils, and the libraries.

which is pretty much standard. The 'all' is not always needed but shouldn't hurt. The install step updates
system directories if you want to install fully, but once you have a binary it should normally just run,

which is pretty much standard. The 'all' is not always needed but shouldn't hurt. The install step updates
system directories if you want to install fully, but once you have a binary it should normally just run,

./bin/xxxxxxx

or some such

Don't distribute such dangerous recipes!
This might end in overwriting system binaries.

And "./bin/xxxxxxx" would start a binary "$CurrentDir/bin/xxxxxxx" so it doesn't match installing into the system's directories.

If a package has a install step, it will install into a sensible place, and remember this is optional

./bin/xxxx
is for running the binary without installing, to test, or most commonly because you have several
versions of something in different directories so you can compare them - you wouldn't normally
install during development for instance.

If a package has a install step, it will install into a sensible place, and remember this is optional

If you want PropGCC and remember someone said it needs TeXinfo4 and then you do "configure/make/install" from TeXinfo4 sources because your OS inly has the newer teXinfo it'll overwrite the system's TeXinfo parts without the system packaging taking notice of this.

And lots more of such examples about what can go wrong with simply trusting "configure/make/install" hints are known.

JUST DON'T DO THIS!
JUST DON'T TEACH THIS!

Find out how to install the wanted software into a path not needing superuser powers.

E.g. 1st make a directory "/opt/parallax" and transfer the ownership of it to an account for managing software with lowered rights and then no further step installing something into "/opt/parallax" will need superpowers.

Look for "PREFIX=...", "--prefix=...", "DESTDIR=..." and other hints how to configure the install path. If all else fails, start to read "Makefile"s. Do not ever rely on such short recipes like: Just type "configure/make/install" with some parts of it as root.

Thay may work by accident or cause accidents that you just do not see...